What are the causes of reduced consciousness?

Causes of Reduced Consciousness

Reduced consciousness results from either global cerebral hypoperfusion (syncope), metabolic/toxic derangements, structural brain lesions, or seizures—with 45-50% having a primary neurological cause and approximately 20% being metabolic or infectious in origin. 1

Primary Classification Framework

The causes of reduced consciousness divide into two fundamental categories 2:

Transient Loss of Consciousness (T-LOC)

• Syncope (cerebral hypoperfusion): Accounts for the majority of transient consciousness loss with rapid, spontaneous recovery 2
• Non-syncope conditions: Include seizures, metabolic disorders, intoxications, and head trauma 2

Persistent/Prolonged Impaired Consciousness

• Structural brain injury, metabolic encephalopathy, or ongoing seizure activity 1

Syncope Causes (Cerebral Hypoperfusion)

Reflex (Neurally-Mediated) Syncope

The most common category, involving inappropriate vasodilation and/or bradycardia 2:

• Vasovagal syncope: Triggered by emotional stress, pain, or prolonged standing 2, 3
• Carotid sinus syncope: Mechanical manipulation of carotid sinuses 2, 3
• Situational syncope: Specific triggers including micturition, defecation, coughing, post-exercise, post-prandial 2, 3

Orthostatic Hypotension

Defined as systolic BP drop ≥20 mmHg or diastolic ≥10 mmHg upon standing 2:

• Primary autonomic failure: Pure autonomic failure, multiple system atrophy, Parkinson's disease with autonomic dysfunction 3
• Secondary autonomic failure: Diabetic neuropathy, amyloid neuropathy 3
• Drug-induced: Antihypertensives, diuretics, vasodilators 2, 3
• Volume depletion: Hemorrhage, dehydration 3

Cardiac Syncope

The most lethal category requiring immediate specialist evaluation 3:

Arrhythmias 2, 3:

• Sinus node dysfunction (sick sinus syndrome)
• Atrioventricular conduction disease (high-grade AV block)
• Paroxysmal supraventricular or ventricular tachycardias
• Inherited arrhythmia syndromes (long QT, Brugada syndrome)
• Pacemaker/ICD malfunction
• Drug-induced proarrhythmias

Structural cardiac disease 2, 3:

• Obstructive valvular disease (aortic stenosis, mitral stenosis)
• Acute myocardial infarction/ischemia
• Hypertrophic obstructive cardiomyopathy
• Atrial myxoma
• Acute aortic dissection
• Pericardial tamponade
• Pulmonary embolism/severe pulmonary hypertension

Cerebrovascular Causes

• Vertebrobasilar TIA: Can cause LOC when accompanied by other neurological signs like paralysis 2, 4
• Subclavian steal syndrome: Vascular steal affecting both brain and arm 2

Non-Syncopal Causes of Reduced Consciousness

Metabolic Disorders 2, 1

• Hypoglycemia: Most common metabolic cause
• Hypoxia: Respiratory failure, carbon monoxide poisoning
• Hyperventilation with hypocapnia
• Electrolyte disturbances: Hyponatremia, hypercalcemia
• Hepatic encephalopathy
• Uremic encephalopathy
• Thyroid disorders: Myxedema coma, thyroid storm

Seizures/Epilepsy 2, 1

Distinguished from syncope by longer duration, post-ictal confusion, and lack of rapid recovery 3

Intoxications 2, 1

• Alcohol
• Sedative-hypnotics
• Opioids
• Anticholinergics
• Drug overdoses

Infectious Causes 1

• Meningitis
• Encephalitis (including COVID-19 encephalitis) 5
• Sepsis with encephalopathy

Traumatic Brain Injury 5, 1

• Concussion
• Intracranial hemorrhage (epidural, subdural, subarachnoid)
• Diffuse axonal injury

Conditions Mimicking LOC (Apparent LOC Without True Unconsciousness) 2

• Psychogenic pseudosyncope: Conversion disorder 2, 3
• Cataplexy: Sudden muscle weakness without LOC 2, 3
• Drop attacks: Sudden falls without LOC 2, 3
• TIA of carotid origin: Does not cause isolated LOC 2

Critical Diagnostic Red Flags

Immediate life-threatening causes requiring urgent intervention 1:

• Hyperacute onset
• Pupillary abnormalities
• Focal neurological deficits
• Meningismus
• Severe headache
• Tachycardia and tachypnea with or without fever
• Muscle contractions/status epilepticus
• Skin abnormalities (petechiae suggesting meningococcemia)

Essential Initial Evaluation

Mandatory for all patients 2, 5:

• Detailed history focusing on circumstances before, during, and after the event
• Witness account when available
• 12-lead ECG
• Orthostatic vital signs (supine and standing BP/HR)
• Focused physical examination including cardiovascular and neurological assessment

Additional testing based on clinical suspicion 5, 1:

• Head CT for trauma or focal deficits
• Basic metabolic panel to exclude hypoglycemia and electrolytes
• Chest imaging if respiratory symptoms present
• Infectious workup if fever/meningismus present

Common Clinical Pitfalls

• Anchoring on psychiatric diagnosis in young patients without completing neurological evaluation 4
• Administering sedating medications before excluding structural brain pathology 4
• Assuming all brief tonic-clonic movements indicate epilepsy: Brief seizure-like activity can occur during syncope 3
• Missing cardiac causes: Failure to obtain ECG or recognize high-risk features 5, 3
• Inadequate neurological monitoring: Standard vital signs insufficient for patients with persistent deficits 4